Almost all North American telephone loop wire pair cable is multipair cable. These transmission circuits are formed by individually twisted pairs of copper wires that are stranded together. In contrast, much of the European and Asian cable plant is constituted of quaded cable or circuits. Some building cables in North America are also quaded. The elemental units in these cables are quads which are four insulated conductors or wires twisted together. The advantage of quading is that more circuits can be packed into a given cross-sectional area of cable. The disadvantage comes from the fact that the quad is geometrically unstable and the capacitances between the conductors in the quad are difficult to control. These factors lead to poorer crosstalk performance between circuits in the same quad.
Quaded cable can be made in two different configurations. In multiple twin quad, the two pairs that constitute the quad are individually twisted and then stranded together to effectively form a two-pair sub-unit. In star-quaded cable, the four conductors are twisted together. Most quaded cable is star-quaded. The cross-section of a typical star-quad is illustrated in FIG. 1A, which shows the four conductors, 1-4, surrounded by insulating sheathings 5-8, respectively. The natural modes of propagation of a perfectly constructed star-quad are obvious from the symmetry of the quad, and are illustrated in FIGS. 1B, 1C, 1D and 1E, where in each of these figures the insulating sheathings are not shown for improved clarity.
In an ideal star-quad, the two balanced pair modes, illustrated in FIGS. 1B and 1C, do not couple to each other because of the symmetry and opposite polarities of two opposite conductors of the quad. The balanced pair modes are frequently referred to as "side circuits," a terminology descended from the use of multiple twin quad. In side circuit one, a first signal is impressed positively and negatively between conductors 1 and 3, respectively, and in side circuit two, a second signal is impressed positively and negatively between conductors 4 and 2, respectively. Besides the balanced pair modes, one could use the phantom circuit of the balanced pair modes as shown in FIG. 1D to transmit a third signal. In the phantom circuit, the third signal is impressed "on-top" of the signals on side circuits one and two, positively and negatively between conductors 1 and 3 together, and conductors 2 and 4 together, respectively. An additional mode of transmission, the ground mode, is shown in FIG. 1E. In this mode a signal is positively impressed on top of all other signals between all four conductors together and an external ground. This mode is not used for transmission because of strong coupling to ground modes in other conductors within the same cable.
The polarities of the voltages on the four conductors guarantees that there would be no coupling between the phantom mode and the side circuits in a perfectly constructed star-quad. Thus, in such a quad, one would get three transmission circuits with tour conductors. As far as coupling between quads is concerned, coupling between side circuits in different quads would be like dipole-dipole coupling as it is in multipair cables. Coupling between phantom circuits would be quadripole-quadripole coupling, which is generally looser than dipole-dipole coupling.
Unfortunately, it is essentially impossible to achieve the precision and stability of the quad structure to take advantage of the mode structure. The mechanical instability of the quad structure leads to coupling between circuits in the same quad. At high frequencies the coupling or cross-talk between conductors can be at unacceptable levels. The far end coupling, the coupling at the receive end of the cable, generated by the signal induced at the transmitted end, increases by 20 dB per decade. Further, the near end coupling induced on the transmitter by the transmitted signals increases by 15 dB per decade.
Consequently, phantom circuits are never used and side circuits in quaded cable have a shorter range than twisted pairs at the higher frequencies used in modern services, such as Basic Rate ISDN.